Bone anchor screws come in a variety of shapes and sizes. One of the more common styles has a polyaxial head that allows for the screw to enter the bone structure at an ideal or preferred inclination. To achieve this polyaxial inclination, the head has a shape configured to allow a complimentary implant device being held by the screw to rotate about its lower external surface. This lower surface can be one of a number of shapes like conical or spherical or hemispherical. This ability is often used in rod receiving implant devices having a modular head assembly.
The modular head pedicle screw assembly generally includes a tulip. A tulip is a body structure having two opposing sides spaced by a slotted opening to receive a spinal rod. The tulip often employs internal threads to receive a rod locking set screw to anchor or fix the rod in the tulip. The lower portion of the tulip has an opening to receive the pedicle screw in a base seat. Often, the tulip can have a saddle that also supports the rod along an underside of the rod. The saddle having an upper recessed curvature into which the rod sits and a lower cup like opening to receive the top of the pedicle screw head. When the saddle and rod and set screw are tightened, the screw angle is fixed against the tulip seat.
Often, it is preferred that the pedicle screw is first placed securely in the bone structure leaving the head protruding above the bone surface. In this surgical procedure the tulip assembly must be adapted to fit down onto the projecting screw head. To accomplish this, the surgeon must push the tulip onto and over the screw head without a clear path of vision. Accordingly, the placement must be accomplished without any way of knowing if the tulip or other device is properly secured. Thereafter, the device is tightened to complete the assembly and the only way to insure the assembly is secure requires an upward pulling of the tightened assembly. This is not a good test because the assembly may be loosened or the screw to bone interface weakened.
It is, therefore, an objective of the present invention to provide a way for a surgeon to place a tulip assembly onto a pedicle screw already threaded into bone in such a way the surgeon can make a proper and secure connection easily.
It is a further objective that the device has properly fitted the tulip assembly onto the pedicle screw head by the very nature of the design. It is another objective that the device provides a self-locking feature that when tightened by assembly, the surgeon can lock the assembly engagement insuring he has made a proper assembly. These and other objectives are achieved by the invention as described hereinafter.